The present invention relates to a rubber composition used as a tread rubber of a pneumatic tire.
Grip performance (including braking performance) and driveability on dry pavement and wet pavement are required in high level in a pneumatic tire, particularly a high performance tire.
In generally, a technique of increasing a compounding amount of a filler and an oil is used to improve grip performance on wet pavement and dry pavement. In such a case, heat build-up and abrasion resistance are decreased. Furthermore, viscosity in an unvulcanized state is elevated and dispersibility of a filler deteriorates. As a result, to obtain sufficient performance, the number of mixing is increased, resulting deterioration of workability. Using a polymer having high glass transition point as a rubber component is considered as other technique. Such a case leads to deterioration of heat build-up and abrasion resistance, and due to deterioration of temperature dependency, deterioration of driveability on dry pavement is induced.
To improve grip performance on wet pavement, silica is generally used. However, when silica is used, workability greatly deteriorates, such as increase of the number of mixing or decrease of extrusion speed (see JP-A-2006-213747 (kokai) and JP-A-2006-160884 (kokai), the entire contents of those references being incorporated herein by reference). To such problems, it is known that a specific silane coupling agent is compounded, and this makes it possible to improve incorporation of silica in a rubber component during mixing and dispersibility of silica while maintaining grip performance on wet pavement and dry pavement, thereby improving workability and heat generation characteristics. However, due to improvement of dispersibility of silica, hardness and dynamic modulus E′ are decreased, and as a result, there is fear of deterioration of driveability on dry pavement.
On the other hand, as a technique to improve driveability on dry pavement, it is considered to increase rubber hardness by, for example, increasing the amount of a filler, decreasing the amount of an oil or adding a hardener. In such a case, grip performance on wet pavement deteriorates.
JP-A-07-90123 (kokai) (the entire contents of this reference being incorporated herein by reference) discloses a rubber composition for a tire tread, having excellent characteristics of abrasion resistance, rolling resistance and wet skid resistance comprising 100 parts by weight of a rubber component comprising from 20 to 50 parts by weight of a natural rubber, from 20 to 40 parts by weight of a styrene-butadiene rubber comprising from 35 to 80% by weight of a vinyl content of a butadiene moiety and from 10 to 40% by weight of a styrene content, and from 20 to 40 parts by weight of an epoxidized natural rubber having a degree of epoxidation of from 10 to 60% by weight, and from 30 to 70 parts by weight of silica compounded with the rubber component.
However, the rubber composition disclosed in JP-A-07-90123 comprises a natural rubber as a main component of a rubber, and the epoxidized natural rubber is used in relatively large amount. Furthermore, the styrene-butadiene rubber used in combination with the epoxidized natural rubber is a rubber having relatively low glass transition point. For this reason, this reference does not disclose to combine a relatively small amount of the epoxidized natural rubber with a specific styrene-butadiene rubber having high glass transition point, and further does not suggest that grip performance on dry pavement and wet pavement and driveability can be improved by such a combination.